Fasteners on commercial and military aircraft carry crucial loads from one component of the aircraft to the next. If the hole is too small for the fastener, the fastener may be difficult to install in the hole, may fit up improperly, or be preloaded in a non-optimal manner. These conditions can cause unwanted noise, create a joint that does not have optimal strength, and increase assembly cost of the aircraft. In addition, the head of the fastener might protrude from the surface of the joint farther than desired. At high aircraft speed, these improperly seated fastener heads would cause increased drag, which would cause additional fuel consumption, thereby necessitating the requirement for additional production steps to correct the condition.
Typically, fastener holes are created after the components to be joined are brought into their final assembly position and clamped together. A tool is then brought into contact with one of the components and advanced through both components to create the hole. The tool is then withdrawn thereby allowing the components to be unclamped for cleaning, chip removal, deburring, and other post-hole formation operations. It is usually desired for the holes to have a sharp edge at the surface of the components to allow the head of the fastener to seat against the component properly. Seated properly, the head of the fastener extends no more than a pre-determined height from the surface of the component.
Ideally, the components are brought back into their final assembly positions with little delay following the post-hole formation activities. The assemblers of the aircraft then measure the diameter of the hole, select an appropriately sized fastener, and fasten the components together. Frequently, though, delays are known to occur between hole formation and final fastener installation. During these delays additional operations may be performed on the components and in the vicinity of the fastener holes. These other operations subject the sharp edge of the hole to wear which causes the edge to become rounded. Additionally, errors and inaccuracies during the formation of the hole may also contribute to the rounding of the edge. It also sometimes occurs that the aircraft will be placed in service with removal of the fastener becoming necessary at some later time (e.g. for maintenance or repairs). If so, it is possible that conditions during the operation of the aircraft may cause additional wear to the hole edge. Also, the maintenance or repair activity may subject the edge of the hole to additional wear and rounding. Because of the rounding of the edge, the resulting measured hole diameter is usually larger than the true diameter of the hole (i.e. the diameter of the hole where the hole is not subject to the wear). The rounded shoulder is sometimes referred to as a “roll-off.” By selecting a fastener in accordance with the diameter as measured at the roll-off, the problems associated with placing a fastener in an undersized hole are created.
A problem closely related to the installation of fasteners in undersized holes is the amount of wear that occurs on measurement standard ring gages used to calibrate “rivet gages.” Rivet gages are rivets manufactured to exacting tolerances. These rivet gages are used to determine how far above the surface of a work piece the head of a rivet (of a pre-selected size) should protrude if inserted into a sharp edged hole of appropriate size. Because, the ring gages used to calibrate the rivet gages are subject to wear and rounding at the edges of the ring gage holes, a rivet gage inserted into a worn ring gage will protrude to a height less than it would in the absence of the wear. As a result, every rivet of the size calibrated with the ring gage will protrude above the work piece farther than expected.
Likewise, the accuracy of the calibration of chamfer gages can suffer from wear and roll off at the edge of the chamfered hole of the chamfer ring gage. The primary difference between calibrating chamfer gages and rivet gages is that the chamfer ring gage used to calibrate the chamfer gage includes a chamfered hole. Nonetheless, the intersection of the chamfered surface and the upper surface of the chamfer ring gage is rounded causing an apparent increase in the outer diameter of the chamfer. As a result, when the chamfer gage being calibrated is placed in the chamfer ring gage, the chamfer gage is set to indicate a larger diameter than it actually measures. As a result, chamfered holes measured with the improperly calibrated chamfer gage are smaller than the measured size indicates. Thus, the fasteners that ought to fit in the chamfer instead protrude from the surface farther than expected. The result is the same as with improperly calibrated rivet gages: increased production costs, increased fuel consumption and less than optimal fit up.
Thus, a need exists for a method of determining the amount of wear, or rounding, that occurs at the edge of a hole.